Geology Reference
In-Depth Information
Therefore, using this simplification, the lifetime of NO
x
is dependent on the
[OH] and [NO]/[NO
2
] ratio. Calculating t
NO
x
under typical upper tropo-
spheric conditions gives lifetimes in the order of 4-7 days and lifetimes in
the order of days in the lower free troposphere. In the boundary layer, the
situation is more complex as there are other NO
x
loss and transformation
processes other than those considered in Equation (2.29), which can make
t
NO
x
as short as 1 h. Integrally linked to the lifetime of NO
x
and therefore
theroleofnitrogenoxidesinthetroposphere is its relation to odd nitrogen
reservoir species, i.e. NO
y
. The sum of total reactive nitrogen or total odd
nitrogen is often referred to as NO
y
and can be defined as NO
y
¼
NO
x
þ
NO
3
þ
2N
2
O
5
þ
HNO
3
þ
HNO
4
þ
HONO
þ
PAN
þ
nitrate aerosol
þ
alkyl nitrate, where PAN is peroxyacetlynitrate (see Section 2.5.4). NO
y
can
also be thought of as NO
x
plus all the compounds that are products of the
atmospheric oxidation of NO
x
.NO
y
is not a conserved quantity in the
atmosphere owing to the potential for some of its constituents (e.g. HNO
3
)
to be efficiently removed by deposition processes. Mixing of air masses may
also lead to dilution of NO
y
. The concept of NO
y
is useful in considering
the budget of odd nitrogen and evaluating the partitioning of NO
x
and its
reservoirs in the troposphere.
16
In summary, the concentration of NO
x
in the troposphere
determines the catalytic efficiency of ozone production;
determines the partitioning of OH and HO
2
;
determines the amount of HNO
3
and nitrates produced; and
determines the magnitude and sign of net photochemical produc-
tion or destruction of ozone (see Section 2.5.2).
2.5.2 Production and Destruction of Ozone
From the preceding discussion of atmospheric photochemistry and NO
x
chemistry, it can be seen that the fate of the peroxy radicals can have a
marked effect on the ability of the atmosphere either to produce or to
destroy ozone. Photolysis of NO
2
and the subsequent reaction of the
photoproducts with O
2
(reactions (2.4) and (2.20)) are the only known
way of producing ozone in the troposphere. In the presence of NO
x
the
following cycle for the production of ozone can take place:
NO
2
þ
hn
O(
3
P)
þ
NO
-
(2.2)
O(
3
P)
þ
O
2
þ
M
-
O
3
þ
M
(2.20)
OH
þ
CO
-
H
þ
CO
2
(2.11)
